1. Field of Endeavor
The present invention relates to a liquid level sensor and more particularly to a tethered liquid level sensor.
2. State of Technology
U.S. Pat. No. 3,409,750 for a liquid level float switch provides the following state of technology information: “A liquid level switch which includes a sleeve made of electrically conducting material which is mounted in a container having a supply of liquid. The bottom of the sleeve is enclosed with an insulating ring which surrounds and separates a metal contact area from the wall of the sleeve in order to form a switch gap in the enclosed bottom area. The metal contact area is a bolt which connects the sleeve, the insulating ring and a drain plug positioned in the bottom of the container together so as to form a unitary device. A ball float is positioned inside the sleeve having at least its surface area covered with a conductive material. The ball normally floats above the bolt and therefore does not bridge the gap between the bolt and the wall of the sleeve. When the liquid level reaches a predetermined low level, the ball float bridges the gap between the bolt and the sleeve to close the switch gap. A further improvement includes a device for detecting an improper placement of the drain plug by energization of an alarm circuit.”
U.S. Pat. No. 4,685,332 for a liquid level sensing device provides the following state of technology information: “A liquid level sensing device utilizing a conductive hollow sphere buoyed by the liquid and guided vertically within a metal tube. An elongated resistance card is mounted in the tube adjacent the sphere to permit the sphere to slide along its surface as the liquid level changes thereby electrically connecting a portion of the resistance card to the metal housing through the conductive sphere. The resistance card is mounted so as to be flexed from its normal flat configuration with the flexed configuration producing a light force on the sphere to maintain good electrical contact therewith and between the sphere and the housing but insufficient to deter the sphere from following liquid level changes.”
U.S. Pat. No. 4,818,976 for a device for monitoring hydrocarbons in groundwater provides the following state of technology information: “An apparatus for sensing the leakage of hydrocarbons from underground storage tanks having a hydrocarbon vapor sensor, a liquid sensor to determine the level of the groundwater and the positioning of the vapor sensor; and, alarm and trouble circuitry to indicate the presence of a hydrocarbon leak or a malfunctioning circuit. The apparatus also contains a timer mechanism to accurately determine whether or not the hydrocarbon has actually leaked from an underground storage tank or is present in the test area due to natural gases or above-ground spills.”
U.S. Pat. No. 5,253,521 for a method of transmitting measured temperature and position parameters from a transducer provides the following state of technology information: “A magnetostrictive liquid level transducer equipped with a number of temperature sensitive resistors is supplied with low level power from a remote station and charges a capacitor to store power for interrogation of the level transducer. The transducer is interrogated periodically for level measurements and the capacitor is recharged between interrogations. During the recharging periods, temperature measurements are made. Thus the level and temperature related data is measured and transmitted alternately. Each measurement results in a pair of pulses having a spacing corresponding to the measured value. The transmission message comprises a plurality of frames or time periods. The first frame is blank and is used for synchronizing the receiving device with the message frames. The subsequent frames have the level and temperature related data interleaved in the message and are easily sorted by the receiving device at the remote station. The data pulses are sent on the power transmission conductors.”
U.S. Pat. No. 5,821,864 for an interface probe apparatus and method provides the following state of technology information: “A wireless interface probe having a transmitter, a signal transport/measuring device, and a receiver as provided. The transmitter has two sensors: a liquid level sensor and a water level sensor. The liquid level sensor determines the boundary between gaseous and liquid media. The water level sensor detects the presence of conductive liquid such as water. Signals generated by the transmitter sensors are transported inductively to the receiver via the signal transport/measuring device. The signal transport/measuring device is a standard metallic measuring tape which affixes to the transmitter via metallic fasteners. The receiver decodes the signals generated by the transmitter and creates an audio and visual reading output. The output reading indicates to the operator the level of the free-phase hydrocarbons and the level of the water. The difference between the two levels is the thickness of the hydrocarbons, which can be determined by using the measuring tape.”
U.S. Pat. No. 7,161,165 for an optical transducer for continuously determining liquid level provides the following state of technology information: “An optical transducer for determining liquid level within a container includes a sensing probe having a first elongate measurement face and a second elongate measurement face that converges toward the first measurement face. A light source is positioned for directing radiant energy into the sensing probe and a photosensor is positioned for detecting an amount of radiant energy exiting the sensing probe. With this arrangement, a change in the amount of radiant energy as detected by the photosensor is indicative of a change in liquid level within the container.”
United States Published Patent Application No. 2008/0060430 for a liquid level measuring device and installation incorporating the same provides the following state of technology information: “A liquid level measurement device comprises a generally seamless pipe for insertion into an installation in which liquid level is to be monitored and a non-contact sensor assembly disposed within the pipe at a location spaced above the liquid. The sensor assembly senses the level of the liquid and generates output representative of the liquid level.”
United States Published Patent Application No. 2009/0301190 for a capacitive sensor assembly for determining relative position provides the following state of technology information: “A capacitive sensor assembly for detecting relative movement or position between objects, such as liquid level in a tank or reservoir, movement of one machine component with respect to another, and so on, includes one or more antenna probes connected to an integrated chip normally associated with touch-screen displays. Each antenna probe operates independently and may be formed as insulated conductive wires or conductive traces between layers of a stiff or flexible substrate, such as a PCB, with the substrate material serving as the insulating layers. Each antenna probe has a different length representing different measurement positions or locations between the objects being measured to provide dynamic calibration of the measurement under changing environmental conditions.”